I. Field of the Invention
The present invention relates generally to wireless communications systems. More particularly, the present invention is directed to a novel system and method for reducing interference generated by digital communications devices.
II. Description of the Related Art
In time-division multiple access (TDMA) cellular communication systems which are well known in the art, the available frequency spectrum is subdivided into a predetermined number of radio frequency channels, each of a given bandwidth. Each radio frequency channel is time-divided into a cycle of TDMA frames and each frame is further time-divided into a cycle of a predetermined number of time intervals, called "slots." Thus, one slot represents a finite time period on a predetermined radio frequency channel. Communication over the radio interface occurs during these slots in groups of modulated bits, called "bursts," with one burst per slot. A "normal burst" contains a packet of bits of useful information which is preceded and followed by a brief "guard period" during which no useful information is nominally transmitted. This guard period allows for power ramping of the transmitter as it turns on prior to transmission of the information bits and turns off after transmission of the information bits.
Although the word "channel" as used above corresponds to a particular fixed radio frequency bandwidth, it should be noted that in the more general case, i.e. in a frequency agile system such as the TDMA based Global System for Mobile Communications (GSM) system, a "channel" may also have a temporal component. That is to say that a "channel" which is dedicated to a single function (i.e. a traffic channel), would be a succession of slots which may occupy different frequencies at different times. It is in this more general, functional sense that the word "channel" will be used below.
When a given subscriber station in the communication system enters a dedicated mode, such as during a call set-up or when performing location updating to the base station, it is typically assigned to a particular slot of a transmit channel for transmission of its information, which has been packaged into a sequence of normal bursts. Thus, although many subscriber stations in a given cell may transmit over a single channel (limited at least by the number of slots in a frame), the individual normal bursts from each user are time-division multiplexed into their corresponding slots. For example, a subscriber station assigned to the first slot in a frame of a given transmit channel will nominally transmit only during the first slot of each frame, and otherwise remains with its transmitter turned off so as to avoid interfering with the burst transmissions of the subscriber stations respectively assigned to the remainder of the slots of that transmit channel frame. Thus, the subscriber station assigned to the first slot will turn on its transmitter during the guard period at the beginning of the first slot, transmit a package of useful information bits during that first slot, turn off its transmitter during the guard period at the end of the first slot, and remain with its transmitter turned off during all other slots of the frame. Likewise, the subscriber station assigned to the second slot of the frame will turn on its transmitter during the guard period at the beginning of the second slot, transmit a package useful information bits during that second slot, turn off its transmitter during the guard period at the end of the second slot, and remain with its transmitter turned off during all other slots of the frame.
This periodic on/off switching of the subscriber station's transmitter produces a transmitted signal which is especially capable of interfering with the operation of nearby electronic equipment. Because this on/off switching modulates the amplitude of the RF energy transmitted, we will refer to interference which is caused by such on/off switching as "amplitude modulation interference", or simply "AM interference." For example, in the pan-European GSM cellular system, each frame has a duration of 4.615 ms, and is divided into eight slots, with each slot being 577 .mu.s in duration. Transmission during the same slot of each frame results in a subscriber station burst repetition rate of 216.6 Hz (i.e., 1/4.615 ms). Since this burst repetition rate is within the audio frequency range, it may be undesirable in the presence of other electronic equipment which contains circuitry which may act as an AM detector. For example, if a GSM subscriber station is operated in the vicinity of a stereo system, the burst repetition rate may be heard as a buzz emanating from the speakers.
It should be noted that other TDMA-based digital communication systems use different frame lengths and a different number of slots per frame. For example, the U.S. TDMA cellular communication system described in Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) Interim Standard 54-B (IS-54-B) uses cyclical TDMA frame lengths of 20 ms, each subdivided into six slots. Also, it should be noted that for higher-bandwidth signals, the TDMA system may assign more than one slot per frame to a single subscriber station in order to accommodate the higher rate signal. However, these systems still use a burst repetition rate in the audio frequency range which may produce substantial interference in surrounding electronic equipment.
Many electronic devices, such as hearing aids and cardiac pacemakers, are also susceptible to interference from such pulsed transmissions. In particular, hearing aids have been found to be sensitive AM detectors, and are designed to provide considerable audio gain. As such, significant interference may be incurred when hearing aids are operated in the vicinity of GSM and other TDMA-type user communication devices. In fact, the level of audio interference or "buzz" in the hearing aid wearer's ear may be enough to drown out the sound of the caller's voice, precluding effective use of a TDMA subscriber station by the hearing aid wearer.
Accordingly, it is an object of the present invention to provide a transmission scheme for a multiple access communication system which minimizes the potential for AM interference with surrounding electronic devices.